The present disclosure relates to methods for electrochemically depositing a conductive material, for example, a metal, such as copper (Cu), cobalt (Co), nickel (Ni) gold (Au), silver (Ag), manganese (Mn), tin (Sn), aluminum (Al), and alloys thereof, in features having a high aspect ratio, such as in a Through Silicon Via (TSV) feature, on a microelectronic workpiece.
TSV deposition is generally directed to creating vertical interconnects through the workpiece for top and bottom connection with interconnects on other workpieces. In one non-limiting example of TSV integration, metal is deposited to fill a TSV via, then the back of the wafer is ground until the bottom of the via is exposed, creating a second connection point for the via. However, it should be appreciated that other types of TSV integrations are also within the scope of the present disclosure.
Typical TSV features have dimensions that may be in the range of about 1 micron to about 15 microns in diameter, and in the range of about 20 microns to about 120 microns in depth. The feature opening is generally large to enable plating to significant depth. Even considering the large opening, TSV features typically still have a very high aspect ratio.
The TSV process may include via etching, insulator and barrier deposition, seed layer deposition, metal filling, and chemical mechanical polishing (CMP). A deposit in a TSV feature may include a dielectric layer, a barrier layer, a seed layer, and a fill layer. In one example, the TSV deposit may include copper in the seed layer, the fill layer, or both.
Because copper tends to diffuse into the dielectric material, barrier layers can be used to isolate the copper deposit from the dielectric material. However, for other metal deposits besides copper, it should be appreciated that barrier layers may not be required. Barrier layers are typically made of refractory metals or refractory compounds, for example, titanium (Ti), tantalum (Ta), titanium nitride (TiN), tantalum nitride (TaN), etc. Other suitable barrier layer materials may include manganese (Mn) and manganese nitride (MnN).
The barrier layer is typically formed using a deposition technique called physical vapor deposition (PVD), but may also be formed by using other deposition techniques, such as chemical vapor deposition (CVD) or atomic layer deposition (ALD). In TSV applications, the barrier layer may typically be about 500 Å to about 4000 Å (about 50 nm to about 400 nm) in thickness.
A seed layer may be deposited on the barrier layer. However, it should also be appreciated that direct on barrier (DOB) deposition is also within the scope of the present disclosure, for example, barriers that are made from alloys or co-deposited metals upon which interconnect metals may be deposited without requiring a separate seed layer, such as titanium ruthenium (TiRu), tantalum ruthenium (TaRu), tungsten ruthenium (WRu), as well as other barrier layers that are known and/or used by those having skill in the art.
In one non-limiting example, the seed layer may be a copper seed layer. As another non-limiting example, the seed layer may be a copper alloy seed layer, such as copper manganese, copper cobalt, or copper nickel alloys. In the case of depositing copper in a feature, there are several exemplary options for the seed layer. First, the seed layer may be a PVD copper seed layer. See, e.g., FIG. 3 for an illustration of a process including PVD copper seed deposition. The seed layer may also be formed by using other deposition techniques, such as CVD or ALD.
Second, the seed layer may be a stack film, for example, a liner layer and a PVD seed layer. A liner layer is a material used in between a barrier and a PVD seed to mitigate discontinuous seed issues and improve adhesion of the PVD seed. Liners are typically noble metals such as ruthenium (Ru), platinum (Pt), palladium (Pd), and osmium (Os), but the list may also include cobalt (Co) and nickel (Ni). Currently, CVD Ru and CVD Co are common liners; however, liner layers may also be formed by using other deposition techniques, such as ALD or PVD.
Third, the seed layer may be a secondary seed layer. A secondary seed layer is similar to a liner layer in that it is typically formed from noble metals such as Ru, Pt, Pd, and Os, but the list may also include Co and Ni, and most commonly CVD Ru and CVD Co. (Like seed and liner layers, secondary seed layers may also be formed by using other deposition techniques, such as ALD or PVD.) The difference is that the secondary seed layer serves as the seed layer, whereas the liner layer is an intermediate layer between the barrier layer and the PVD seed. See, e.g., FIGS. 5 and 6 for illustrations of processes including secondary seed depositions, followed by, respectively, ECD seed deposition in FIG. 5, as described below, and flash deposition in FIG. 6. (A “flash” deposition is primarily on the field and at the bottom of the feature, without significant deposition on the sidewalls of the feature.)
In TSV applications, the seed layer may typically be about 2000 Å to about 8000 Å (about 200 nm to about 800 nm) in thickness. It can be challenging to reliably deposit a seed layer on the sides and bottom of the via (particularly using the PVD technique) as a result of the high aspect ratio of the via. In that regard, discontinuities in the seed layer often result, which can cause typical defects such as bottom-sidewall voids and pinch-off in the via.
After a seed layer has been deposited according to one of the examples described above, the feature may include a seed layer enhancement (SLE) layer, which is a thin layer of deposited metal, for example, copper having a thickness of about 1000 Å (100 nm). An SLE layer is also known as an electrochemically deposited seed (or ECD seed). See, e.g., FIG. 4 for an illustration of a process including PVD seed deposition and ECD seed deposition. See, e.g., FIG. 5 for an illustration of a process including secondary seed deposition and ECD seed deposition. As seen in FIGS. 4 and 5, ECD seed may be a conformally deposited layer.
An ECD copper seed is typically deposited using a basic chemistry that includes a very dilute copper ethylenediamine (EDA) complex. ECD copper seed may also be deposited using other copper complexes, such as citrate, tartrate, urea, etc., and may be deposited in a pH range of about 2 to about 11, about 3 to about 10, or in a pH range of about 4 to about 10.
After a seed layer has been deposited according to one of the examples described above (which may also include an optional ECD seed), conventional ECD fill and cap may be performed in the feature, for example, using an acid deposition chemistry. Conventional ECD copper acid chemistry may include, for example, copper sulfate, sulfuric acid, methane sulfonic acid, hydrochloric acid, and organic additives (such as accelerators, suppressors, and levelers). Electrochemical deposition of copper has been found to be the most cost effective manner by which to deposit a copper metallization layer. In addition to being economically viable, ECD deposition techniques provide a substantially bottom up (e.g., nonconformal) metal fill that is mechanically and electrically suitable for interconnect structures.
Conventional ECD fill, particularly in features having a high aspect ratio, like TSV features, has proven to be difficult. For example, the high aspect ratio of the feature and discontinuities in the seed layer greatly increase the chances of pinch-off at the top of the feature and bottom-sidewall void formation in the via. To avoid pinch-off and void formation in the via, conventional ECD fill in a TSV via is typically a slow process because of the amount of metal required to fill the TSV via, sometimes taking hours to partially fill the via, and still proving to be difficult for fill because of void formation in the via.
Therefore, there exists a need for an improved feature filling process for a high aspect ratio feature, for example, a TSV feature.